The nature of lipid-lipid interactions at interfaces is being studied so as to better understand such interactions in biological systems where they greatly influence physiological and pathological processes, e.g. in biological membranes and serum lipoproteins. The interfacial properties of a large number of fatty acids, differing in chain length, geometric isomerism, presence of double, triple bonds, and hydroxy groups and the number of such groups, are being studied with the insoluble monomolecular film technique. Surface tension changes for these films at different surface concentrations (number of molecules per unit area) are used to estimate the various thermodynamic parameters associated with the interfacial interactions occurring between lipids in single component systems and mixtures. Treatment of mixtures as two-dimensional solutions allows one also to determine the degree of miscibility, and hence homogeniety within the surface film. Chain characteristics giving rise to different levels of phase separation are correlated with similar behavior noted in biological systems. BIBLIOGRAPHIC REFERENCES: "The Effect of Ethylenic and Acetylenic Groups on the Properties of Fatty Acid Monolayers, H. L. Welles, G. Zografi, C. M. Scrimgeour and F.D. Gunstone. "Monolayers", Advances in Chemistry Series, No. 144, ed. E. D. Goddard, 1975. pp. 135-152. "Wettability of Pharmaceutical Solids: Estimates of Solid Surface Polarity," G. Zografi and S.S. Tam. J. Pharmaceutical Sciences, 65, 1145-1149 (1976).